Recently, with developments on digitalization of electric devices and higher-speed personal computers, capacitors, downsized but having large capacitance and low impedance in a high-frequency region, are demanded, and there have been proposed solid electrolytic capacitors using electroconductive polymer having electron conductivity as solid electrolyte.
Generally, a basic element (6) of a solid electrolytic capacitor is produced by forming a dielectric oxide film layer (2) on an anode substrate (1) consisting of a metal foil having a large specific surface area increased by etching treatment as shown in FIG. 1, forming thereon a solid semiconductor layer (3) (hereinafter referred to as solid electrolyte) as a counter electrode, and desirably further forming a conductor layer (4) of conductive paste. Generally, in order to ensure insulation between solid electrolyte (3) (cathode part) and anode substrate (1), a masking layer (5) is further provided therebetween and then electrodes are added as appropriate.
Generally, a conductor layer (4) has a double-layer structure including a carbon paste layer in the inner side (to the electrolyte layer side) and a silver paste layer in the outer side. In order to suppress the equivalent series resistance (ESR) to a low level, it is desirable to reduce resistance of a cathode electrode including a conductor layer. Generally, since resistance in a carbon paste layer is larger than that in a silver paste layer, the ratio of the carbon paste layer against the silver paste layer may be reduced so that the resistance of the cathode electrode can be lowered. For example, in Patent Document 1 (Japanese Patent Application Laid-Open No. H11-135377) proposes that a conductor layer be formed only of a conductive metal layer such as silver paste layer.
However, since silver migrates in the electric field, a conductive path may be formed between a cathode and an anode due to the migration in a case where a carbon paste layer is not used as in the above patent document, which may lead to an increase in leakage current (LC).
In addition, recently, reduction in capacitor thickness has been demanded and to meet this demand, it is desirable that thickness of a cathode electrode be reduced to the minimum. However, there is a problem that reduction in a cathode electrode, especially in a silver paste layer leads to an increase in ESR.
As above, there are requirements conflicting with each other in configuration of a cathode electrode. Therefore, a cathode electrode where reduction in thickness, low ESR, low LC and the like are achieved at the same time has been demanded.    [Patent Document 1] Japanese Patent Application Laid-Open No. H11-135377